Merging upstream version 1.68.2+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 40 insertions, 0 deletions

diff --git a/tests/ui/higher-rank-trait-bounds/issue-46989.rs b/tests/ui/higher-rank-trait-bounds/issue-46989.rs
new file mode 100644
index 000000000..4a09f4be1
--- /dev/null
+++ b/tests/ui/higher-rank-trait-bounds/issue-46989.rs
@@ -0,0 +1,40 @@
+// Regression test for #46989:
+//
+// In the move to universes, this test started passing.
+// It is not necessarily WRONG to do so, but it was a bit
+// surprising. The reason that it passed is that when we were
+// asked to prove that
+//
+//     for<'a> fn(&'a i32): Foo
+//
+// we were able to use the impl below to prove
+//
+//     fn(&'empty i32): Foo
+//
+// and then we were able to prove that
+//
+//     fn(&'empty i32) = for<'a> fn(&'a i32)
+//
+// This last fact is somewhat surprising, but essentially "falls out"
+// from handling variance correctly. In particular, consider the subtyping
+// relations. First:
+//
+//     fn(&'empty i32) <: for<'a> fn(&'a i32)
+//
+// This holds because -- intuitively -- a fn that takes a reference but doesn't use
+// it can be given a reference with any lifetime. Similarly, the opposite direction:
+//
+//     for<'a> fn(&'a i32) <: fn(&'empty i32)
+//
+// holds because 'a can be instantiated to 'empty.
+
+trait Foo {}
+
+impl<A> Foo for fn(A) {}
+
+fn assert_foo<T: Foo>() {}
+
+fn main() {
+    assert_foo::<fn(&i32)>();
+    //~^ ERROR implementation of `Foo` is not general enough
+}